Applications for the use of lithium batteries are growing rapidly. However, problems of cycle life, which is the number of charge/discharge cycles a battery may undergo, as well as problems of calendar life, namely the period of time a battery system is operative, have limited the use of lithium batteries in high energy applications such as hybrid and fully electric vehicle applications. In many instances, lithium batteries employ carbon, and in particular graphite, based anodes; and these anodes can be a significant factor in lowered cycle life and calendar life of lithium batteries in which they are incorporated, owing to exfoliation and other physical degradation of these materials during their operational life.
For purposes of this disclosure the terms “batteries” and “cells” will be used interchangeably when referring to one electrochemical cell, although the term “battery” can also be used to refer to a plurality of electrically interconnected cells. A generalized lithium battery includes an anode and a cathode which are disposed in a volume of a nonaqueous electrolyte material, which typically includes one or more lithium salts and a solvent such as an organic carbonate material. In most instances, the anode and cathode have a body of separator material interposed therebetween. During the charging of the battery, lithium ions travel from the cathode to the anode and are intercalated therein. During discharge of the battery, the process reverses. During the initial charging of the battery, the surface of the anode can react with lithium ions and components of the electrolyte to form a layer of material referred to as a “solid electrolyte interface” (SEI) layer. It should be noted that in some instances this SEI layer is also referred to as a “solid electrolyte interphase” layer. The thus produced SEI material is electrically insulating but conductive of lithium ions. The reaction is irreversible and consumes some of the lithium capacity of the battery.
Particular problems arise in connection with graphite and other carbon based anodes since the SEI layer produced during their charging can cause exfoliation of the carbon surface. This exfoliation degrades the integrity of the body of carbon thereby shortening its cycle and calendar lives. Furthermore, such degradation also removes the previously formed SEI layer necessitating reformation during subsequent charge cycles thereby consuming lithium and further decreasing the charge capacity of the battery.
As will be appreciated, stabilization of the SEI layer on carbon based anodes will greatly increase their cycle and calendar lives. In some instances, the prior art has looked to the use of composite material such as two phase materials, coated particles, nanoscale composites, three-dimensional microstructures, and the like in an attempt to stabilize carbon-based anode materials. Such approaches are complex and expensive to implement, and have met with limited commercial success.
Given the problems with stabilizing carbon based anodes, the prior art has also investigated alternative, non-carbon based anode structures. For example, silicon is capable of intercalating large amounts of lithium; however, in doing so it undergoes very large changes in volume, which leads to pulverization and degradation of anode structures. The art has sought to accommodate these volume changes by utilizing composite and/or multiphase structures which include buffer materials. These structures are difficult to implement, and also decrease the amount of lithium which can be intercalated. In another approach thin films of silicon have been proposed as the active component of lithium battery electrodes. While thin films of silicon are not prone to pulverization, the actual amount of silicon they contain is very small and thus their charge capacity per unit area is correspondingly small. Hence, the art is still looking to find methods and materials for stabilizing the surfaces (and associated SEI layers) of carbon based anodes.
As will be explained in detail hereinbelow, the present invention provides a method and structure for stabilizing carbon, and in particular graphite, based anodes of lithium batteries. The methods and materials of the present invention may be implemented utilizing well developed, high volume thin film deposition techniques so as to produce anode structures having an SEI layer which is not only stabilized, but also optimized so as to provide maximum performance. These and other advantages of the invention will be apparent from the drawings, description, and discussion which follow.